But if the gene is recessive, the game changes a lot -- it would mean that two magical parents would always have magical children, no matter what their background may be. All squibs would have to be mutants, and many generations of ordinary muggles could carry a recessive magical gene that might eventually pop up and make a muggle-born wizard-baby. This includes Harry's mom, meaning that Harry himself could have easily been a normal, boring English kid for whom the closest thing to a magical adventure was huffing glue.
The Harry Potter Lexicon
Harry may have gotten screwed when it comes to eyesight, but he was lucky otherwise.
Regardless of which theory is correct, though, Lord Voldemort looks like an idiot. He doesn't want to mix pure-bloods (aa) with muggle-borns (also aa), but a pairing of that type would inevitably produce a magical child (aa again) if the gene is recessive, so it makes no damn sense. And if the gene is dominant, then there's still an excellent chance that the child will turn out to be a wizard even if the wizard parent marries an actual muggle (to say nothing of a muggle-born). So, like most racists, Voldemort is a moron -- probably something to do with that filthy muggle blood in his veins.